The present invention relates to an optical system capable of retrieving information from an optical memory by means of focused light beams.
In general, in an optical system used in an optical head of an information retrieving apparatus, a laser beam emitted from a semiconductor laser is collimated by a collimator lens. Then, the collimated beam is passed through a polarized beam splitter and a quarter-wave plate, and focused on an optical memory, having a spiral or concentric track, by an objective lens. The laser beam, reflected by the optical memory, is returned to the beam splitter via the objective lens and the quarter-wave plate. Resultant light beams, reflected by the splitter to be transferred to the memory, are directed toward different optical paths. The reflected laser beams from the splitter are converged on a photodetector by a projection lens, to be detected by the photodetector. Thus, information is read from the optical memory, a focusing state of the objective lens is detected, and whether or not a desired track of the memory is traced correctly with the focused laser beam, is determined.
Examples of the prior art optical system of this type are disclosed in U.S. Pat. Nos. 4,521,680; 4,585,933 and 4,546,460, by Ando.
In the conventional optical system, the laser beam directed to the optical memory, and the laser beam, reflected from the memory and directed toward the photodetector, are transferred along the same optical path, between the polarized beam splitter and the memory. However, different optical paths lie between the splitter and semiconductor laser and between the splitter and photodetector, therefore, the semiconductor laser can next be located not to the photodetector. Thus, the prior art optical system cannot readily be reduced in size. Moreover, the convex collimater lens is located between the beam splitter and semiconductor laser, and the convex projection lens is located between the splitter and photodetector. Thus, two different lenses are needed separately for the beam collimation and beam convergence. As a result, the optical system is inevitably heavy in weight, so that the access time, required in retrieving specific data by moving the system, is longer than those of magnetic disk apparatuses. As compared with the minimum access time of about 30 ms for the magnetic disk apparatuses, the access time of optical information recording/retrieving apparatuses is as long as about 100 ms.